ByJoseph C. HarschMarch 25, 1988

If you stop for a moment to think about it, what happened in Washington this week was amazing. Yet relatively little attention was paid to it. ``It,'' in this case, is the fact that Soviet Foreign Minister Eduard Shevardnadze arrived in Washington, spent a lot of time with United States Secretary of State George Shultz on the first day, went the next day to the White House where he had an easy, comfortable chat with the President, and the two spent the evening watching ``That's Entertainment.''

It was serious business in a courteous and friendly context. What is more important, it seemed natural. Such a visit in such an easy atmosphere was inconceivable during the first five or six years of the Reagan administration. In this, the eighth year, it happens as a normal routine event.

Which does not mean that there are no longer problems between the two superpowers. There are big problems and they are not easy to resolve. But they are being tackled with a mutual desire to find solutions where possible, and each with respect for the positions of the other.

In other words, the foreign-policy leaders of the superpowers met over the past week not to try to score propaganda points off each other, but to try to resolve mutual problems. According to top US State Department officials, they were making remarkable progress.

For example, it is reported that Messrs. Shevardnadze and Shultz already have a private understanding about how to get around the question of continued supplies to their respective clients in Afghanistan as Soviet troops pull out. The two sides could not reach a formal, signed agreement. But it is understood that the US will watch the flow of Soviet arms to the Kabul regime and will pattern its flow of weapons to the Afghan resistance forces accordingly.

Similar ways of doing business are having their effect in respect to Nicaragua. There is no written agreement between Americans and Soviets about the amount of aid that Moscow will continue to funnel to the Ortega regime in Nicaragua. But this week, President Daniel Ortega Saavedra's brother was actually sitting across a table from contra delegates and discussing a cease-fire.

The contra rebels were at the peace table partly because the US Congress has cut off any more lethal aid. The Sandinistas were there largely because Moscow has put a lid on the aid it sends them. There is no official or signed agreement between Moscow and Washington about these matters. It is a matter of private ``understandings'' between Messrs. Shultz and Shevardnadze.

That is the way serious diplomacy often works - quietly, tacitly, behind the scenes, and without formal treaties or public announcements.

An interesting feature of the tacit Afghan and Nicaraguan deals is that, in effect, both the US and Soviet Union are giving up on the attempt to impose a puppet regime upon a small neighbor.

The Russians since czarist times have wanted to control Afghanistan. The US loves to have ``friendly'' governments in all of Central America. The Soviets planted a puppet regime in Afghanistan with Soviet troops (the only way it could be done). The US has attempted to plant a regime of its own choosing, the contras, in Nicaragua. Neither project has worked out.

One end result will be an Afghanistan more independent of Moscow and probably largely neutral between the two superpowers. Another possible end result will be a Nicaragua coming to a compromise agreement with the political opposition, but not becoming a complacent client of Washington under a regime recruited by the Central Intelligence Agency.

Not all problems between big powers and small neighbors are susceptible to solution by private understanding between Moscow and Washington. Washington this week was wrestling with one such problem in Panama.

That small country of large importance has fallen into the control of a native, and locally popular, ``strong man'' with acquisitive habits. He has been indicted in US courts on charges of massive involvement in the illegal drug trade. He is known to be a very wealthy man. Washington wants him out.

Gen. Manuel Antonio Noriega is a capitalist. But in his personal combat with Washington he has turned for help to Fidel Castro of Cuba. A planeload of guns was reported this week to have arrived in Panama from Cuba. There is no evidence that Moscow had anything to do with this. But it is symptomatic of the behavior of small neighbors of large countries down through the ages.

The small country seeking independence from its big neighbor almost reflexively turns to the remote great power. The Eastern European captives of Moscow look, at times, toward the US for help. The Afghan resistance looked to the US. The Sandinistas looked to Moscow.

Meanwhile Messrs. Shultz and Shevardnadze were also this week still hoping to be able to tie up an agreement on strategic weapons for their masters to sign in Moscow in May. But, no matter what, Ronald Reagan is to see the towers of the Kremlin May 29 to June 2, in person, while he is still President, and whether there is an arms treaty to crown the occasion or not.